Foam extrusion technology is well known in the art. Traditional expanded polystyrene technology has existed since the early 1950's. Uses for low density foam extrusions include foam insulation and food packaging.
Presently, tandem extruders are typically used in the production of extruded foam boards or rods. The first extruder is for melting and mixing the blowing agent. The second extruder is for cooling the melt. See, for example, Modern Plastics Encyclopedia, 1982-1983, pp. 274-275. Such technology works well with commodity thermoplastics, e.g., polystyrene, but for engineered thermoplastic, the high viscosity in the melt precludes free translation of the processing parameters, especially extrusion temperatures and pressures.
Limitations in the use of polystyrene for the manufacture of foam extrusion have been found to include poor temperature stability and low compressive strength when low densities are used, due in part to large cell size and poor orientation. Increased compressive strength is an important feature especially for foam insulation because it is often the physical property of the product which prohibits lowering the density of the foam even further to reduce material costs.
Although many foamed articles comprising engineering thermoplastics, such as polycarbonates, polyesters, polysulfones, polyphenylene ethers, as well as compositions with other thermoplastics, such as polystyrenes are known, it is not generally possible to readily produce closed foamed structures easily from them, especially at densities of about 20 lbs./cu. ft. and particularly with compressive strengths above about 20 psi. Krutchen et al, U.S. Pat. Nos. 4,598,101 and 4,598,104, deal with foamable compositions of polyphenylene ethers and high impact rubber modified styrenes. Only a specific narrow class of blowing agent were used, namely, liquid chlorinated hydrocarbons. One example, methylene chloride is found to form an insoluble complex with polyphenylene ether at room temperature. These have the drawback of not producing optimum foams apparently because they act as solvents for the resins both at room temperature and at the elevated temperatures employed in the extruders. In addition, the blowing agents used by Krutchen et al are believed not to be suitable for expanding resin mixtures containing crystal polystyrene.
A need therefore, continues to exist to produce articles from engineered thermoplastics with compressive strengths above about 20 psi and especially above about 50 psi to about even above 100 psi, and methods to make such, and the foamed articles themselves have now been discovered and are the subject matter of this invention. It is important in securing these results to use a blowing agent which is essentially insoluble in the resin at room temperature but soluble at elevated processing temperatures, such as a hydrocarbon or a chlorofluorocarbon.
It is therefore an object of the present invention to provide low density extrusion foam product with closed cell structure and high compressive strength.
It is a further object of the present invention to provide flame retardant embodiments of such foams.